Molecular rotational spectroscopy is a technique that offers high chemical selectivity and sensitivity and can be used to analyze gas samples, such as mixtures. Techniques for broadband spectroscopy, especially in the field of molecular rotational spectroscopy, offer significant improvements in measurement time and detection sensitivity as compared to other approaches. For room-temperature samples, a peak of the spectral intensity of a rotational spectrum typically occurs in the range of millimeter-wave (“mm-wave”) frequencies (e.g., from about 60 gigahertz (GHz) to about 1000 GHz), particularly for molecules with 2-10 “heavy” nuclei (non-hydrogen atoms). A molecular rotational spectrum of most molecules will contain multiple, spectrally narrow transitions in any fixed mm-wave frequency range of modest bandwidth (e.g., a bandwidth of about 30 to about 50 GHz). Broadband techniques can be useful in cases where the power available from light sources exceeds a threshold to saturate the molecular transitions. For rotational spectroscopy of low pressure gases (such as in molecular beams or static gases) the power meeting a threshold for saturation is often orders of magnitude lower than the power provided by generally-available microwave-to-terahertz (THz) light sources.